1. Field of the Invention
The present invention relates to an electrochromic device (hereinafter referred to as ECD) which is applicable produced with a plasma polymerization process.
2. Description of the Prior Art
An ECD is used for a solid-state passive display the color of which is changed when positive or negative charges are injected thereto. It comprises a coloring layer of the above-mentioned material and a layer of ionic conductor, which are interposed between two opposing electrodes.
The materials used as an ionic conductor layer are classified into three groups: liquid electrolytes, solid electrolytes and dielectrics. In an ECD which makes use of a liquid electrolyte such as sulfuric acid or lithium perchlorate/propylene carbonate, a complete sealing structure against the liquid is needed in order to prevent leakage or freezing of the liquid, so that its usage is limited. On the contrary, an ECD of all solid type which makes use of a solid electrolyte, especially a macromolecular solid electrolyte not only solves the above-mentioned problem, but also can be used in a wide temperature range, with a high reliability, and further can be driven easily by a matrix drive means (for example, U.S. Pat. No. 4,456,337). Thus, an ECD of all solid type has been developed.
The solid electrolyte used in an ECD of all solid type can be classified into three kinds: a ionic conductor of metallicions or a material wherein metallic ions are dispersed, a ferrodielectric film, and a macromolecular solid electrolyte. Especially, macromolecular solid electrolytes have been attracted attension and have been studied because of excellent shapability.
An ECD of all solid type which uses Nafion (registered trade mark of a polymer ion-exchange resin of perfluorosulfonic acid which owned by du Pont chemicals) as an electrolyte layer has already been developed. However, Nafion can be prepared in a form of a thin film only down to 250 .mu.m. Further, the response time of Nafion is very slow to an order of about one second because H.sup.+ ions absorbed in macromolecules need a long time to migrate therethrough. An ECD of all solid type which makes use of a mixed macromolecular solid electrolyte is reported to have the contrast and the response time of the same orders of those of an ECD liquid electrolyte type. The mixed macromolecular solid electrolyte is a material which is prepared by mixing an electrolyte with Teflon powder and a white pigment and by compacting the mixed material.
On the other hand, an ECD which makes use of a macromolecular complex solid electrolyte has also been developed (for example, Japanese Patent laid open publication No. 40531/1983 and No. 135431/1984.) The macromolecular complex solid electrolyte is prepared by dispersing an inorganic salt and/or a plasticizer in a macromolecular polymer. The addition of the plasticizer makes ions to migrate very easily so that the electrolyte operates effectively as a solid electrolyte in an ECD. An ECD which uses this type of electrolyte has the response time of for example 0.2 sec and the contrast of almost the same order as those of a prior art ECD of liquid electrolyte type, while it has excellent shapability. Thus, it can be used in various uses.
Further, an ECD which makes use of an organic solid electrolyte film has also been proposed in order to improve the operation characteristics of ECD (for example, Japanese Patent laid open publication No. 30731/1983). The ECD of this type is advantageous, from a viewspoint of the production thereof, in that it is easy to be produced that a variety of ECDs can be produced according to needs thereof and that the production cost can be reduced.
The coloring layer of an ECD can be composed of either one of two types of electrochromic (EC) materials: an inorganic compound and an organic compound. Among inorganic compounds, some inorganic compounds of translation metals, especially WO.sub.3, have been developed for an coloring layer. However, an EC material of transition metal oxide shows only one color generally, and the tone of color is not necessarily clear. Accordingly, it is hard to display a full color image. On the contrary, EC materials of organic compounds are known to be superier in the property of coloring.
EC materials of organic compounds can be classified with respect to the coloring mechanism into a reversible electrodeposition type and an ion injection type. As an EC material which belongs to the former type, there are known viologen dyes and anthraquinone dyes, while tetrathiafluvalene (TTF) and rare earth diphthalocyanines (U.S. Pat. Nos. 4,432,612 and 4,456,337) are known as an EC material of the latter type. A deposition film of rare earth diphthalocyanines, for example, of lutetium diphthalocyanine changes its color from red through orange, green, blue to violet according to the applied electric voltage in a range between +1.5 V to -1.5 V. Thus, multicoloring can be realized by using one kind of EC materials of the latter type, so that it is hopeful that a multicoloring ECD can be commercialized practically.
However, there remain many problems to be solved for a practical use. Especially, such problems have been pointed out that the repetition life is short, that the response rate is slow and that the contrast is low.
As for the repetition life, each of the EC layer, the electrodes and the electrolyte is caused to deteriorate, respectively, and the life is of an order about 10.sup.7 repetitions at maximum in a practical level. If the repetition life is improved to an order of 10.sup.8, the ECD of this type will be utilized in a variety of uses, so that the repetition life of an order of 10.sup.8 is a target level to be attained. It is made clear that the deterioration of the EC layer is caused by the capture and storage of alkali metallic ions of the electrolyte layer (regardless of a liquid or a solid) in the EC layer in the process of repeated going in and out of the ions of the EC layer. It is also one of reasons which cause the deterioration that Joule heat generated by an electric current applied upon the driving accelerates a chemical reaction so that a color deposition is caused in the EC layer to make the coloring dull.
As to the slow response rate, it is known that the response rate can be improved for organic EC materials down to several tens msec (for example 50-100 msec). However, the response rate of 10 msec is required to be attained for a practical use. The slowness of the response may be ascribed to the slowness of the mobility in the electrolyte layer. It is also affected by the dimension of the contact area between the electrolyte layer and the coloring layer.
As to the low contrast, the coloring density can be made higher by increasing a voltage to be applied. However, this makes the repetition life short and causes the display electrodes to deteriorate. Therefore, in a view point of practical use, it is not appropriate to increase the voltage to be applied. The increase in the porosity of an EC film is also one of solutions for improving the contrast, and the increase in the deposition pressure is effective for the increase in the porosity. However, this lowers the mechanical strength of the EC film due to the electrochemical reaction between the EC electrode and the electrolyte, so that the life becomes short. In other words, chemical changes such as elution and deposition of the electrolyte layer become large as far as the electrolyte layer is arranged in an ECD.